I've heard claims ranging from "higher-quality lenses only make a noticeable difference for scientific applications and super-huge prints," to "even the difference between high-quality and uber-high-quality lenses can be seen with the naked eye on 6x10 prints"

Before I sell my house so I can take amazing pictures from a cardboard box, I'd like to actually see the difference. However, all the examples I've found compare photos not only from different lens, but also with different cameras/scenes/photographers!

Are there any examples of the same scene, with the exact same settings on the same camera, taken with multiple lenses (of differing quality/price)? Multiple comparisons with different scenes/cameras (especially cameras of different qualities) would be preferable, to eliminate any variables. I'd also prefer a non-biased source (so, not the lens-manufacturer).

I'd also be interested in comparing telephoto to zoom+teleconverter (of the same quality, focal-length etc.), or comparing zoom to fixed-length... but perhaps those are asking too much :)

Keep in mind that lenses vary significantly more than camera bodies and even on sites like photozone or digital-picture, I have seen different results which both coincide and are completely different from my observations. I frequently see multiple copies of the same lens (sometimes dozens) and it shows that differences between identical models can be great.
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ItaiApr 21 '13 at 1:24

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Part of the problem with insisting on identical settings is that you eliminate the areas where the higher quality lens can take a picture the other lens can't. An f/5.6 lens can not take a properly exposed photo if the ISO and Tv needed require f/2.8. Nor can an f/5.6 or f/2.8 lens get a desired shallow depth of field of 4" at 5 feet with a 50mm lens, but an f/1.4 can. The best the f/2.8 lens can do is 6". The best an f/5.6 lens (like the typical 18-55mm kit lens) can do is 12".
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Michael ClarkApr 21 '13 at 1:36

I'm not sure you'll find one site that compares all lenses with one realistic scene, but a few lens review sites I've found interesting in the past include

Ken Rockwell's Camera and Lens Reviews Has reviews of high-end Canon and Nikon lenses, and he's not afraid to tell you to buy the cheaper models if he doesn't see any real difference in image quality (and he offers photographic evidence to make his case).

Pentax lenses don't feature hugely on these sites, but they might illustrate whether top-end lenses really offer enough extra quality to justify their considerably higher price. And Ken Rockwell's site will probably just tell you to save your money and spend it on going out to find things worth photographing anyhow.

Note that Ken also "reviews" equipment he's never used. He has good information and interesting opinions, but it's mixed in with a lot of junk, which he excuses by comparing his own site to satirical newspaper The Onion.
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mattdmApr 24 '13 at 11:52

Sure; this is exactly what what http://photozone.de does in lens reviews. The sample images aren't always exactly the same but show similar subjects, but the technical analysis is all done for each lens on the same camera (for each brand).

For example, the sample images for Pentax's 70mm DA Limited and earlier 77mm FA Limited show statues in the same garden from roughly the same viewpoint, but they're taken in different days and without mechanical precision. However, the technical analysis, done on an optical bench and shown on the previous pages are done in a controlled environment.

If you want super-identical images, reality limits that to still-life studio setups; other Lens review sites like SLR Gear will have those; see for example the the Pentax 40mm DA Limited and 43mm FA Limited. You can see both a studio still-life and a test target. These images, by their nature, are pretty dull and can't really tell you the whole story of the lens.

The Digital Picture site linked to in other answers also shows test target images, without even a studio still-life. You may find DPReview's Lens Comparator Widget similarly helpful; it gives a colorful presentation of results and you can hover over parts of the image for actual crops from the test target. But really, I can't see much advantage of staring at test targets yourself over looking at the crunched data and reading what the reviewers are saying.

As I re-read your question, though, I think that what you're actually asking may be best covered by What characteristics make a good lens good?, which goes into what exactly the practical, visible differences are between higher-quality lenses and what you might want to look for in any samples. Once you know what you're looking for, it's not really important for the test images to be exactly identical.

I have found several sites that have pictures from different lens (such as pixel-peeper) - however, that is only helpful if most of the other variables (camera, lighting, aperture, focal length, shutter time, movement of subject, distance to subject, etc. etc.) are the same in both images, which is rarely the case.
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BlueRaja - Danny PflughoeftApr 17 '13 at 13:41

Re Edit: That link is very helpful, thank you :) But, I am looking for specific examples with specific lens, eg. I know that chromatic aberration is reduced in higher quality lenses, but I'd like to see exactly how bad the CA is in the cheaper lenses, and how much it is reduced in the more expensive anti-dispersion ones, so I can decide whether or not it's worth the cost for me
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BlueRaja - Danny PflughoeftApr 17 '13 at 13:48

Since CA in particular is something measurable, you can see that numerically in many of the reviews, and examples of it in test-target shots.
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mattdmApr 17 '13 at 13:50

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One other aspect that's missing: differences between lenses may be "seen with the naked eye on 6x10 prints" precisely because the more expensive lens allows things which the cheaper lens doesn't. You can't compare a cheap 50-200mm f/3.5-f/5.6 zoom with a 70-200mm f/2.8 lens at 200mm @ f/2.8. So, in some ways, you're pre-stacking the deck by saying: "I'd like to see if a Trabant gets me there as quickly as a Porsche when driving below the speed limit down a busy city street."
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mattdmApr 17 '13 at 14:24